Engineering photonic environments for two-dimensional materials

In the last decade, the rise of two-dimensional (2D) materials has attracted a tremendous amount of interest for the entire field of photonics and opto-electronics. The mechanism of light–matter interaction in 2D materials challenges the knowledge of materials physics, which drives the rapid development of materials synthesis and device applications. 2D materials coupled with plasmonic effects show impressive optical characteristics, involving efficient charge transfer, plasmonic hot electrons doping, enhanced light-emitting, and ultrasensitive photodetection. Here, we briefly review the recent remarkable progress of 2D materials, mainly on graphene and transition metal dichalcogenides, focusing on their tunable optical properties and improved opto-electronic devices with plasmonic effects. The mechanism of plasmon enhanced light–matter interaction in 2D materials is elaborated in detail, and the state-of-the-art of device applications is comprehensively described. In the future, the field of 2D materials holds great promise as an important platform for materials science and opto-electronic engineering, enabling an emerging interdisciplinary research field spanning from clean energy to information technology.

Atomically thin two-dimensional (2D) materials have been a famous and fascinating material in recent years due to the potential to replace conventional semiconducting bulk electronic materials. To control the performance of 2D materials, many methods have been proposed, including physical and chemical ways, to manipulate the electronic, atomic and microscopic properties. In this work, we would like to present a physical method based on the interactions of 2D materials with light to influence the 2D material properties and device performance. By reviewing some recent published work, we will show how effective the light can be to functionalize 2D materials. The fundamental fluorescence phenomenon and current applications using 2D materials in optoelectronics, such as photodetectors, solar cells and light emitting diodes, to obtain improved device properties will also be discussed.

Two-dimensional (2D) materials such as graphene, transition metal dichalcogenides and phosphorene have emerged as brand new photonic materials with huge potential in a variety of optical applications in recent years, due to their unique properties and multiple functions. Owning to the significantly increased surface to volume ratio in the atomic thin layered structure, the interactions between 2D materials with light become remarkably strong in either elastic or inelastic interactions. Most of previous studies were focused on the inelastic interactions between 2D materials with light, namely Raman and photoluminescence properties. However, we found the elastic interactions between them and light are also very attractive. Based on the properties discovered, we demonstrated several optical applications, such as atomically thin optical lenses and gratings. On the other hand, we found the inelastic interactions with light in the 2D limitation affected a lot by the external environment, which could serve as a unique method to control the working status of the optical devices to meet multiple application needs. Several progresses were demonstrated such as control of excitons and trions in few-layer MoS2 via the co-modulations by both temperature and electric field and the defect engineering by oxygen plasma etching to stimulate new photon emission.

Enhanced optical absorption in two-dimensional (2D) materials has recently moved into the focus of nanophotonics research. In this work, we present a gain-assisted method to achieve critical coupling and demonstrate the maximum absorption in undoped monolayer graphene in the near-infrared. In a two-port system composed of photonic crystal slab loaded with graphene, the gain medium is introduced to adjust the dissipative rate to match the radiation rate for the critical coupling, which is accessible without changing the original structural geometry. The appropriate tuning of the gain coefficient also enables the critical coupling absorption within a wide wavelength regime for different coupling configurations. This work provides a powerful guide to manipulate light–matter interaction in 2D materials and opens up a new path to design ultra-compact and high-performance 2D material optical devices.

Current understanding and emerging applications of 3D crumpling mediated 2D material-liquid interactions

The 2D materials based on the layered structures, from graphene, to transition metal dichalcogenides (TMDs), to black phosphorous and few-layer topological insulators have emerged as novel platforms for both fundamental studies and device applications. Here, we review the progress in microcavity-enhanced light-matter interaction in 2D materials. In particular, we focus our attention on the devices based on microcavities and photonic crystals embedded with graphene and TMDs. Effects observed under the weak and strong coupling regimes are discussed followed by brief overview of future directions.

Over the last decade, two-dimensional (2D) materials have drawn much attention for the next-generation electron devices, optoelectronics and bioelectronics [1, 2]. Recently, non-volatile resistance switching (NVRS) behavior has been observed in various 2D materials, including graphene oxide, functionalized MoS2 and partially degraded black phosphorus, etc. [3-5], where the resistance can be modulated between a high-resistance state (HRS) and a low-resistance state (LRS) [6]. In addition, there are several reports about NVRS in multi-layer hexagonal boron-nitride (h-BN) [7]. Although 2D materials are widely regarded as promising candidates to overcome vertical scaling obstacle in NVRS based on conventional metal-insulator-metal (MIM) structure [3], few researchers believe that single-layer 2D material can produce NVRS [8] due to excessive leakage current. Sangwan et al. discovered that grain boundaries in single-layer MoS2 can produce NVRS based on planar (horizontal) structure [9]. However, the planar structure without 3D stacking ability has the limitation of low integration density. Recently, we reported the discovery that NVRS phenomenon is accessible in a variety of single-layer transition metal dichalcogenides (TMDs) in vertical MIM structure [10, 11]. Herein, we report another 2D insulating material, single-layer h-BN, also shows stable and desirable NVRS behavior. Compared with monolayer TMDs with three atomic layers, monolayer h-BN has only one atomic layer and ~0.33 nm in thickness, which is the thinnest active layer in non-volatile resistance memory. These devices can be labelled as “atomristor”, which means the memristor effect in atomically thin nanomaterials.[12] The h-BN atomristors have been studied using a crossbar structure (Au/h-BN/Au) [See Figure a]. Figure b shows a representative I-V curve of h-BN atomristor, demonstrating forming-free switching with large on/off ratio (up to >106) and low switching voltage (down to <1V). In addition, pulse operation proves its fast switching speed (<15 ns), which is comparable to the state-of-the-art in 2D memory. Ab-initio simulation reveals that the metal ion substitution in sulfur vacancies may be related to the switching from HRS to LRS, and area scaling test result is also consistent with this conductive-bridge-like mechanism. The results of this work indicate a potential universal resistive switching behavior in 2D insulating monolayers, which is applicable to memory technology, neuromorphic computing, RF switch and flexible electronics.

2D materials (2DMs), due to spin-valley locking degree of freedom, exhibit strongly bound exciton and chiral optical selection rules and become promising material candidates for optoelectronic and spin/valleytronic devices. Over the last decade, the manifesting of 2D materials by circularly polarized lights expedites tremendous fascinating phenomena, such as valley/exciton Hall effect, Moiré exciton, optical Stark effect, circular dichroism, circularly polarized photoluminescence, and spintronic property. In this review, recent advance in the interaction of circularly polarized light with 2D materials covering from graphene, black phosphorous, transition metal dichalcogenides, van der Waals heterostructures as well as small proportion of quasi-2D perovskites and topological materials, is overviewed. The confronted challenges and theoretical and experimental opportunities are also discussed, attempting to accelerate the prosperity of chiral light-2DMs interactions.

The development of two‐dimensional (2D) materials is experiencing a renaissance since the adventure of graphene. 2D materials typically exhibit strong in‐plane covalent bonding and weak out‐of‐plane van der Waals interactions through the interlayer gap. Opening 2D materials is an effective way to alter the physical and chemical properties, such as band gap, conductivity, optical property, thermoelectric property, photovoltaic property and superconductivity. A larger interlayer distance means more accessible active sites for catalysis, an ion‐accessible surface in the interlayer space, which may greatly enhance the performance of 2D materials for energy conversion and storage. Moreover, opening 2D materials by intercalation can change the band filling state and the Fermi level. This review mainly focuses on the opening of 2D materials and their subsequent applications in energy conversion and storage fields, expecting to promote the development of such a new class of materials, namely expanded 2D materials. The exciting progresses of these expanded materials made in both energy conversion and storage devices including solar cells, thermoelectric devices, electrocatalyst, supercapacitors and rechargeable batteries, is presented and discussed in depth. Furthermore, prospects and further developments in these exciting fields of the expanded 2D materials are also commented.

The layered two-dimensional (2D) materials, e.g., the well-known graphene, transition metal dichalcogenides, and topological insulators, have attracted great interest of researchers from fundamental research as well as industries owing to their intriguing properties in a number of aspects. In practical applications, the efficient modification of the features of 2D materials is compelling and essential to achieve desired functionalities. Ion implantation has been successfully applied to synthesize graphene. The most exotic advantage of ion beam technology is that it offers distinct options of energetic beams, which has recently shown the unique capability to modify and tailor the properties of versatile 2D materials. To name a few, the energetic ion beams could implement the surface morphology or layer-to-layer structural engineering of 2D materials. At the microcosmic level, the introduction of ion beam induced defects and intentional doping of specific ions are the basis of tailoring properties of 2D materials. By manipulating the parameters of ion beams (energies, species, fluences, incident angles, etc.), the modified 2D materials may possess novel properties, which are unprecedented in pristine ones. Promising applications based on these 2D materials with ion beam tailored features have been realized in a broad range of fields. In this work, we systematically review the latest research progress on the deployed techniques and property modifications of graphene and other 2D materials under the treatment of various ion beams. A few selected applications are presented to indicate the practical potentials of ion beam modified 2D materials in distinct areas. Perspectives on future developments are also provided by focusing on several promising topics.

Symbolic regression for defect interactions in 2D materials

Atomically thin sheets of materials, the so-called two-dimensional (2D) materials arrived on the scene in early 2000s with the successful isolation of graphene as freestanding monolayer films. Graphene, a single atom thick sheet of sp2-hybridized carbon bonded in a honeycomb lattice, has exceptional electronic, optical, mechanical and thermal properties that outperform those of most of the existing materials. The research on graphene further fuelled emergence of related 2D materials such as black phosphorous, silicene, germanene and transition metal dichalcogenides (TMDs) such as MoS2. Due to their exotic properties, these materials offer virtually endless opportunities for fundamental research as well as cutting edge applications. Most of these 2D materials however exist as single layers only when supported by another solid surface or when stabilized by physisorbed or chemisorbed organic molecules. Given their layered nature, most 2D materials are extremely difficult to disperse in typical solvents. Chemisorption of organic molecules onto their basal plane allows dispersion of these materials in (organic) solvents thereby improving their solution processability. Such dispersions can be used in composite materials and as functional inks. Moreover, covalent functionalization allows modification of the intrinsic electrical, electronic and optical properties of these 2D materials. In this talk, I will discuss covalent modification graphene, graphite and MoS2 using diazonium chemistry. Two different routes for reductive decomposition of diazonium salts namely, chemical and electrochemical, will be discussed in detail. Special focus is on sub-nanometer characterization of modified materials using scanning tunneling and atomic force microscopy (STM and AFM). Initial results towards patterned covalent functionalization using physisorbed self-assembled templates will also be presented besides the discussion on the use of such surfaces as seed layers for atomic layer deposition (ALD).

The fascinating two-dimensional (2D) materials are being unconsciously applied in various fields from science to engineering, which is benefited from the glamorous physical and chemical properties of mechanics, optics, electronics, and magnetism. The representative 2D thermoelectric/piezoelectric materials can directly convert thermal/mechanical energy into electrical energy, which can resolve the energy issues and avoid further environmental deterioration. The thermoelectric or piezoelectric properties of various 2D materials, such as graphene, hexagonal boron nitride, black phosphorus, transition metal dichalcogenides (TMDs), arsenene, metal carbides and nitrides (MXenes), and so on, have been investigated in detail. Although tremendous progress has been achieved in the past few years, these properties still need to be improved for their practical application by designing new 2D materials, strain engineering, chemical functionalization, etc. In addition to this, in 2D materials, there are many other novel physical properties, such as magnetism, topology, valley, and so on. The combination of thermoelectricity/piezoelectricity with other unique properties may lead to novel device applications or scientific breakthroughs in new physics. Overall, the emergence of 2D thermoelectric and piezoelectric materials has expanded energy conversion research dramatically. By combing this new device concept with the novel 2D materials, original devices should have potential applications in energy harvesting.

Two-dimensional materials have garnered interest from the perspectives of physics, materials, and applied electronics owing to their outstanding physical and chemical properties. Advances in exfoliation and synthesis technologies have enabled preparation and electrical characterization of various atomically thin films of semiconductor transition metal dichalcogenides (TMDs). Their two-dimensional structures and electromagnetic spectra coupled to bandgaps in the visible region indicate their suitability for digital electronics and optoelectronics. To further expand the potential applications of these two-dimensional semiconductor materials, technologies capable of precisely controlling the electrical properties of the material are essential. Doping has been traditionally used to effectively change the electrical and electronic properties of materials through relatively simple processes. To change the electrical properties, substances that can donate or remove electrons are added. Doping of atomically thin two-dimensional semiconductor materials is similar to that used for silicon but has a slightly different mechanism. Three main methods with different characteristics and slightly different principles are generally used. This review presents an overview of various advanced doping techniques based on the substitutional, chemical, and charge transfer molecular doping strategies of graphene and TMDs, which are the representative 2D semiconductor materials.

Two-dimensional (2D) materials have brought a spectacular revolution in fundamental research and industrial applications due to their unique physical properties of atomically thin thickness, strong light-matter interaction, unity valley polarization and enhanced many-body interactions. To fully explore their exotic physical properties and facilitate potential applications in electronics and optoelectronics, an effective and versatile characterization method is highly demanded. Among the many methods of characterization, optical second harmonic generation (SHG) has attracted broad attention because of its sensitivity, versatility and simplicity. The SHG technique is sufficiently sensitive at the atomic scale and therefore suitable for studies on 2D materials. More importantly, it has the capacity to acquire abundant information ranging from crystallographic, and electronic, to magnetic properties in various 2D materials due to its sensitivity to both spatial-inversion symmetry and time-reversal symmetry. These advantages accompanied by its characteristics of non-invasion and high throughput make SHG a powerful tool for 2D materials. This review summarizes recent experimental developments of SHG applications in 2D materials and also provides an outlook of potential prospects based on SHG.